Process for the direct production of cast iron from iron bearing ore, and an apparatus suitable to carry out said process

ABSTRACT

A process is described for direct production of cast iron starting from iron bearing ore in an apparatus having two communicating chambers in which to carry out the process, comprising the following operations: pre-reduction and pre-heating to founding point of the iron bearing ore in a first substantially cylindrical chamber; final reduction, carburization and founding of the resulting iron in a second chamber arranged below said first chamber, provide the reducing gas both in said second and in said first chamber, provide the reducing gas both in said second and in said first chamber, characterized by the fact that: said iron bearing ore and oxygen are introduced into said first chamber through the side walls thereof, simultaneously but separating, the oxygen being introduced at a speed lower or equal to the speed of introduction of said iron bearing ore; and said oxygen, coal and flux are introduced into said second chamber simultaneously but separately through the side walls thereof and in a manner inclined downwards and towards the center of said second chamber.

DESCRIPTION

The present invention relates to a process and an apparatus for thedirect production of cast iron and, more specifically, to a process andan apparatus for the production of cast iron starting from iron bearingore (hereinafter also indicated as "iron ore") of varying grain size andin various stages of oxidation.

Processes for the indirect production of cast iron from carbon coke andiron bearing ore made to undergo agglomeration are well known and testedand currently represent the only industrial process for production ofcast iron in which the production capacity exceeds one million tons peryear.

Processes for direct production of cast iron from non-coke carbon andiron bearing material which are not made to undergo agglomerationprocesses, and in which both materials have a low grain size, are onlyknown on a pilot test level.

In particular, processes are known that have two stages.

A first stage for pre-reduction and pre-heating to a temperature atwhich the iron bearing ore starts to melt which takes place in asubstantially cylindrical chamber, into which the iron bearing ore isdriven by means of a plurality of nozzles in such a way as to give it arotational motion along the walls of the chamber, thus increasing thetime that it remains in contact with a reducing gas, for example carbonmonoxide, supplied from the lower chamber.

And a second stage for final reduction of the pre-reduced matterobtained in the first chamber, which takes place in a second chamberpositioned below the first one and in communication therewith, andnormally referred to as a converter chamber, into which a fuel, forexample fine coal, and an element supporting combustion, for exampleoxygen, are input to create a hot reducing gas capable of reducing thecompound coming from the first pre-reduction chamber in a smeltedcondition through delivery of the gas produced in the firstpre-reduction and pre-heating chamber.

In the first chamber the energy required by the chemical reactions andphysical transformations that take place is provided by post-combustionof the reducing gas, generated by gasification of the coal, with theoxygen injected into the converter and the oxygen bound to the iron ore.

According to a method that forms part of the state of the art is knownthe use of a central nozzle for emission into the second chamber(converter) of fuel and oxygen.

The use of the central nozzle in the converter reduces the uniformity ofdistribution of the coal and oxygen with respect to the overall surfacearea of the metal bath at which these raw materials are aimed.

Furthermore, according to a method foreseen in the European Patent No.EP 0 690136, the central nozzle solution can cause problems in long-termoperation of the cyclone; in effect this nozzle can become an obstacleto the uniform performance of the chemical reactions and create asurface on which deposits can form, resulting in partial blockage of theworking area.

For this reason, in spite of the progress made to date in this field,the problems relating to processes for the direct and continuousproduction of cast iron from iron bearing ore, and the identification ofapparatuses suitable to carry out said processes and capable of givinghighly flexible production, remain substantially unsolved.

The aim of the present invention is therefore to provide a process forthe direct and continuous production of cast iron and an apparatussuitable to carry out said process, said apparatus not involving the useof central nozzles, and being capable of guaranteeing a high level ofproduction flexibility.

This aim is achieved by the present invention by means of the use ofside nozzles to inject coal, flux and oxygen into the converter.

In fact in this way there is a greater uniformity of distribution ofsaid materials throughout the cross section of the converter, moreeffective stirring of the slag which in this way increases the reductionkinetics and the heat exchange between slag/bath/gas, and greaterflexibility in the post-combustion levels that can be achieved in theconverter.

The object of the present invention is a process for the directproduction of cast iron starting from iron bearing ore, in an apparatushaving two chambers in communication with one another in which to carryout the process, comprising the following operations:

pre-reduction and pre-heating to smelt of the iron bearing ore in afirst substantially cylindrical chamber, in which the iron bearing orreand oxygen are introduced through the side walls thereof;

final reduction carburizing in a second chamber followed by melting ofthe resulting iron from the chamber arranged below said first chamber,in which coal and oxygen, injected into said second chamber, provide thereducing gas both in said second and in said first chamber,

characterised by the fact that:

said iron bearing ore and oxygen are introduced into said first chamberthrough the side walls thereof, simultaneously, the oxygen beingintroduced at a speed lower or equal to the speed of introduction ofsaid iron bearing ore; and

said oxygen, coal and flux are introduced into said second chambersimultaneously, but separately through nozzles in the side walls thereofand in a manner inclined downwards and towards the centre of said secondchamber.

Furthermore, the present invention provides an apparatus for the directproduction of cast iron starting from iron bearing ore, comprising:

a first chamber having a substantially cylindrical shape;

means for supplying iron bearing ore to said first chamber;

first means for supplying oxygen to said first chamber;

a second chamber having a substantially cylindrical shape, with adiameter greater than that of the first chamber, arranged underneathsaid first chamber and in communication therewith by means of aconnector in the shape of a truncated cone;

second means for supplying oxygen to said second chamber;

means for supplying fuel and flux to said second chamber;

third means for supplying oxygen to said second chamber; and

a gas outlet pipe connected with the top part of said first chamber;

characterised by the fact that said second means supplying oxygen tosaid second chamber are made up of nozzles inclined downward and equallyspaced one from the other along the perimeter of said second chamber.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE enclosed shows an axonometric projection of anapparatus according to the present invention.

So far, a general description has been given of the present invention.With the aid of an example a more detailed description of a preferredembodiment of the invention will now be given, with the aim ofclarifying the aims, characteristics, advantages and operating methodsthereof.

EXAMPLE

The apparatus is made up of a first cylindrical body 1 and a secondcylindrical body 2, arranged underneath the first body 1 and connectedto the latter by means of a truncated cone connector 3. The cylindricalbody 1 is connected at the top to an outlet pipe 4 (partiallyillustrated in the FIGURE) which has the job of connected at the top toan outlet pipe 4 (partially illustrated in the FIGURE) which has the jobof transferring the gas produced during the process to the outside ofthe apparatus.

For the sake of simplicity, the first cylindrical body 1 will beindicated in the following as the "cyclone" and the second cylindricalbody 2 will be indicated as the "converter".

The cyclone 1 has two injection levels, one for iron ore and one foroxygen.

With reference to the FIGURE, injection takes place through injectorsspaced at an equal distance from one another, of which two (5 and 6) areshown in the FIGURE.

Underneath the truncated cone part 3 is the converter, in which finalreduction to iron of the iron ore that has been pre-reduced in thecyclone takes place, as does carburization and smelting of the ironproduced. In the converter fine coal, flux and oxygen are injected(indicated in the following for the sake of clarity as "primaryinjection") by means of a number of tuyeres 8, the number and positionsof which are dependent on the size of the converter (only two of whichare illustrated in the FIGURE). The level at which the tuyeres arepositioned corresponds to a suitable position above the liquid castiron, in order to avoid re-oxidation of the bath and to obtain a highlevel of post-combustion.

Also provided, by means of downwards inclined nozzles 7, is theinjection of additional oxygen (indicated in the following for thepurpose of clarity as "secondary injection") at a higher level than thatof the tuyeres 8, for post-combustion of the gas.

With the aim of improving heat exchange and mass exchange, the inlet ofmixing gas is provided from the bottom of the converter by means ofporous plugs (not illustrated in the FIGURE).

The liquid cast iron, together with the slag, is then tapped through thepipe 9.

The characteristics of the embodiment of the apparatus according to theinvention used in this example are indicated in the following table.

                  TABLE 1                                                         ______________________________________                                        Converter                                                                     ______________________________________                                        D: internal diameter         2     m                                          H: height (including truncated cone connector)                                                             5.8   m                                          Ratio H/D, between total working height (H) and internal                                                   2.9k-                                            ing diameter of the cylindrical part (D)                                      Ratio d/D, between internal diameter of cyclone and internal                                               0.6                                              diameter of converter                                                         Ratio h/H, between height of cyclone and height of converter                                               0.8                                              Truncated cone connector                                                      Height                       2.3   m                                          Angle of inclination of the connector from converter to                                                    78°                                       Ratio between height of truncated cone connector and height                                                0.4                                              converter                                                                     ______________________________________                                    

The use of this apparatus allows improvement of the emulsion of coal,gas, slag and liquid metal with a consequent improvement of heatexchange between the gaseous phase (CO, H₂ produced by partialgasification of the coal) and the metal bath, and of mass exchangebetween the pre-reduced matter, liquid cast iron and gas.

The process is characterised by two main stages, which take placeseparately in the two chambers of the reactor and which are:

pre-reduction and pre-heating of the iron ore in the cyclone to thefounding point; and

final reduction, carburization and founding into liquid cast iron of thepre-reduced iron ore in the converter.

The iron mineral (or iron bearing ore) is input into the cyclonetogether with the carrier gas that transports it. Simultaneously, oxygenis blown into the cyclone.

Furthermore, a gas coming from the underlying converter is also fed intothe cyclone.

This gas is made up of CO, CO₂, H₂, H₂ O and N₂ and has a temperature of1600-1800° C.

The CO and the H₂ in the gas partially reduce the iron bearing ore(pre-reduction), while the gas as a whole contributes towardspre-heating it.

The oxygen blown into the cyclone reacts with the gas from theconverter. This combustion serves to provide the energy necessary tosupport the reduction reaction and to encourage pre-heating of the ironbearing ore.

The level of pre-reduction and pre-heating obtained are measured by twobasic process parameters:

a) the pre-reduction degree (PRD); and

b) the temperature of the pre-reduced matter.

By expressing the iron oxide in the pre-reduced matter as FeOx, the PRDis defined as follows:

    PRD=(1-×/1.5)×100

    ______________________________________                                        in which:                                                                            x =    1.5     that is to say Fe.sub.2 O.sub.3                                                           PRD =   0%;                                        x =    1.33    that is to say Fe.sub.3 O.sub.4                                                           PRD =  11%;                                        x =    1       that is to say FeO                                                                        PRD =  33%.                                 ______________________________________                                    

The pre-reduced and pre-heated iron bearing ore, in a semi-molten state,passes as a result of gravity into the converter.

It must be taken into account that not all the iron bearing ore ispre-reduced. Part of it is drawn out of the cyclone by the gas leavingthe plant through the outlet pipe (4).

In the converter there is a bath of liquid iron and a liquid slag phaseon top of the iron.

Final reduction of the pre-reduced matter takes place in the slag phase.Reduction takes place by means of fine coal injected directly into theslag and by means of the CO that is produced both by the reductionreaction and by direct gasification of the coal.

The reactions that result in final reduction of FeO to Fe can besummarised as follows:

    FeO+C=Fe+CO                                                (I)

    FeO+CO=Fe+CO.sub.2                                         (II)

    2C+3/20.sub.2 =CO+CO.sub.2                                 (III)

The coal gasification reaction (III) is the result of stabilisation ofthe chemical balance between the oxidised and reduced iron and coalproducts in the liquid metal-slag interface.

Following this balance and following the presence of CO₂, primarypost-combustion takes place in the slag phase.

By blowing oxygen into the converter through the top nozzles 7 secondarypost-combustion takes place, which serves to provide energy to reducethe iron bearing ore and to maintain the temperature of the moltenproducts at a suitable level.

Post-combustion is achieved by blowing oxygen through the side walls ofthe converter into the layer of slag lying on top of the metal bath.

This injection takes place radially on two levels, in the first levelthrough tuyeres (8) equipped with concentric coaxial nozzles through theinner part of which the solid mixture of fine coal and flux is fed, andin the second level through nozzles (7) positioned at a higher level.

This arrangement ensures that oxygen is introduced into the converter inthe direction of the surface above the bath of molten metal,corresponding to the overlying slag, thus avoiding any interactionbetween the stream of oxygen and the metal bath.

The parameters describing post-combustion are the primary and secondarypost combustion ratio (PCR), in which the PCR is defined as the ratio ofthe concentrations of oxidised chemicals to the chemicals oxidised andreduced by gas:

    PCR=(CO.sub.2 +H.sub.2 O)/(CO+CO.sub.2 +H.sub.2 +H.sub.2 O).

The energy produced as head by post-combustion of the gas is transferredwith a certain level of efficiency to the liquid phases in which it isrequired.

The efficiency of energy transfer is known as the Heat TransferEfficiency (HTE), where HTE is:

1 less the ratio between the excess of energy remaining in the gas aftercombustion and all the energy that can be transferred as heat.

The gas produced by final reduction and by the two post-combustionphases leaves the converter and enters the cyclone, where it performspre-reduction and pre-heating of the iron bearing ore.

The iron ore and coal fractions that do not react go to form the slagphase (foam-like top portion and compact underlying portion) whichfloats on top of the molten iron.

The amount of slag, which influences the reduction kinetics, and thequality of the slag, which influences the composition of the cast ironand its adhesion to refractory surfaces, is controlled by means ofaddition in the converter of specific fluxes such as CaO and MgO.

The input of mixing gas, for example nitrogen, is provided from thebottom of the converter, by means of porous plugs.

The following is an example of an embodiment of the process for directproduction of cast iron starting from iron bearing ore and of theapparatus suitable to carry out said process. The specifications of theembodiment of the process according to the invention used in the exampleare summarised in the following table.

                  TABLE 2                                                         ______________________________________                                        Post-combustion ratio of gas leaving the cyclone (PCR)                                                  80%                                                 Post-combustion ratio of gas leaving the converter (PCR)                                                40%                                                 Pressure                  3 bar                                               Heat Transfer Efficiency from the gas to the solid under                                                80%                                                 fusion (cyclone HTE)                                                          Heat Transfer Efficiency from the gas to the metal bath                                                 80%                                                 (converter HTE)                                                               Cyclone PCR/HTE           0.4                                                 Converter PCR/HTE         0.4                                                 Cyclone (water cooled)    temperature                                                                   1700-2000° C.                                Converter (refractory)    temperature                                                                   1500-1700° C.                                ______________________________________                                    

Table 3 indicates the materials entering (in kg/h for the solids, Nm³ /hfor the gas and temperatures in ° C.), whereas Table 4 indicates thematerials leaving the apparatus.

                  TABLE 3                                                         ______________________________________                                        Cyclone                                                                       Iron bearing ore                                                                           1512 kg/t cast iron                                                           quality: fine iron ore with high Iron content                    tertiary oxygen                                                                            200 Nm.sup.3 /t cast iron                                        Converter                                                                     coal         640 Nm.sup.3 /t cast iron                                                     grain size: 70 micron                                            coal carrier gas                                                                           v = 100 m/s                                                      primary oxygen                                                                             100 Nm.sup.3 /t cast iron                                                     v = 250 m/s                                                      secondary oxygen                                                                           300 Nm.sup.3 /t cast iron                                                     v = 250 m/s                                                      flux         100 kg/t cast iron                                                            grain size: 1 micron                                             mixing gas   700 Nm.sup.3 /h (nitrogen)                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Cyclone                                                                       process gas  1400        Nm.sup.3 /t cast iron                                Converter                                                                     cast iron    1000        kg                                                   slag         250         kg                                                   ______________________________________                                    

Table 5 indicates the working dimensions required to carry out theprocess correctly, in terms of chemical reactions and heat exchange.

                  TABLE 5                                                         ______________________________________                                        Liquid bath mixing power                                                                              1.5 KW/t cast iron                                    Ratio between Mass of cast iron and Mass of slag in                                                   2                                                     the converter                                                                 Ratio between Height of foaming top layer of slag                                                     2.7                                                   and Height of compact slag                                                    Ratio between Volume of gas in the slag and Total                                                     0.6                                                   Volume of foaming slag                                                        ______________________________________                                    

What is claimed is:
 1. A process for the direct production of cast ironstarting from iron bearing ore in an apparatus having two communicatingchambers in which to carry out the process, comprising the followingoperations:pre-reduction and pre-heating to smelt the iron bearing orein a first substantially cylindrical chamber, in which iron bearing oreand a first supply of oxygen are introduced through the side wallsthereof; final reduction carburizing in a second chamber arranged belowsaid first chamber, followed by melting of the resulting iron, whereincoal and a second supply of oxygen are injected into said second chamberto provide the reducing gas, whereinsaid iron bearing ore and firstsupply of oxygen are introduced into said first chamber through the sidewalls thereof simultaneously, the first supply of oxygen beingintroduced at a speed lower or equal to the speed of introduction ofsaid iron bearing ore; and coal and flux are introduced into said secondchamber through chutes in the side walls thereof, and at least part ofsaid second supply of oxygen is introduced into said second chambersimultaneously with but separately from said coal and flux throughnozzles in the side walls thereof at a level higher than said chutes andin a manner inclined downwards and towards the center of said secondchamber.
 2. A process according to claim 1, in which said coal has agrain size of between 0.05 and 3 mm.
 3. A process according to claim 2,in which said coal is injected into said second chamber at a speedequivalent to at least 80 m/s.
 4. A process according to claim 1, inwhich said oxygen is injected into said second chamber at a speedequivalent to at least 100 m/s.
 5. A process according to claim 1, inwhich said fluxes have a grain size of below 2 mm.
 6. A processaccording to claim 1, in which the ratio between the volume of gas inthe slag and the total volume of foaming slag is at least 0.4.
 7. Aprocess according to claim 1, in which the ratio between the mass ofcast iron and the mass of slag is between 2.5 and 4.5.
 8. A processaccording to any claim 1, in which the ratio between the Post CombustionRatio (PCR) and the Heat Transfer Efficiency (HTE) is between 0.3 and0.5.
 9. A process according to claim 1, in which the mixing power in thebath is lower than 2.5 kW per ton of cast iron.
 10. A process accordingto claim 1, in which the height of the foaming layer on top of the slagis at least 2.0 times the height of the compact slag.
 11. A processaccording to claim 1, in which the oxygen introduced into said secondchamber is directed towards the surface overlying said molten metalbath, corresponding to the overlying slag.
 12. An apparatus for directproduction of cast iron starting from iron bearing ore, comprising:afirst chamber having a substantially cylindrical shape; means forsupplying iron bearing ore to said first chamber; first means forsupplying oxygen to said first chamber; a second chamber having asubstantially cylindrical shape, with a diameter greater than that ofthe first chamber, arranged underneath said first chamber and incommunication therewith by means of a connector in the shape of atruncated cone; second means for supplying oxygen to said secondchamber; means for supplying fuel and flux to said second chamber; thirdmeans for supplying oxygen to said second chamber; and a gas outlet pipeconnected with the top part of said first chamber; characterised by thefact that:said second means supplying oxygen to said second chamber aremade up of nozzles inclined downward and equally spaced one from theother along the perimeter of said second chamber and at a level higherthan said means for supplying fuel and flux to said second chamber. 13.An apparatus according to claim 12, in which said means for supplyingiron bearing material to said first chamber are made up of injectornozzles arranged around the circumference of said first chamber andequally spaced one from the other.
 14. An apparatus according to claim13, in which said nozzles supplying iron bearing material are arrangedin an inclined position with respect to the radial direction of thefirst chamber.
 15. An apparatus according to claim 12, in which saidsecond means supplying oxygen to said second chamber are made up ofinjector nozzles arranged around the circumference thereof and equallyspaced one from the other.
 16. An apparatus according to claim 15, inwhich said injector nozzles supplying oxygen are arranged in an inclinedposition with respect to the radial direction of the second chamber. 17.An apparatus according to claim 16, in which said nozzles are inclineddownward with respect to the horizontal by an angle of betweenapproximately 10° and 40°.
 18. An apparatus according to claim 12, inwhich the ratio between the total height of the second chamber and theinternal diameter thereof is between 1.4 and 3.2.
 19. An apparatusaccording to claim 12, in which the ratio between the internal diameterof the first chamber and the internal diameter of the second chamber isbetween 0.4 and 0.8.
 20. An apparatus according to claim 12, in whichthe ratio between the height of the first chamber and the height of thesecond chamber is between 0.4 and 1.2.
 21. An apparatus according toclaim 12, in which the ratio between the height of the truncated coneconnector and the height of the second chamber is between 0.3 and 0.5.22. A process according to claim 1 wherein part of said second supply ofoxygen to said second chamber is fed simultaneously with said coal andflux through said chutes.